You transform into a specific alternate form of a common Small or Medium humanoid ancestry, which you choose when you gain this feat. Most nymphs typically choose an elf or a human form. This form is the same age and body type as your true form and has roughly analogous physical traits, such as hair color. Using Mortal Shape counts as creating a disguise for the Impersonate use of Deception. You can remain in your alternate form indefinitely, and you can shift back to your nymph form by using this action again.